In the nearly half century since the introduction of the long-playing low-noise vinyl phonograph record launched the high fidelity home music system industry, and the more than a quarter century since the stereophonic disc stimulated that industry to its present size and sophistication, despite all the improvements in the technology, even the best of home music systems does not yet carry the conviction of reality, the sense that the listener is present at an actual performance. Music as reproduced in the home is always recognized as sound emanating from loudspeakers. This failure in the listener's perception of reality can be ascribed to the fact that prior art loudspeaker systems, even those considered the best and most expensive, do not quite achieve a true sense of reality.
The failure to obtain the aural ambience of a live performance from sound fields reproduced in the home is due largely to a high level of directionality inherent in the design of prior art loudspeakers. This is a problem which is more acute as the bandpass of audio systems becomes better. The unquestioning acceptance of this shortcoming of prior art loudspeaker systems is due to the generally accepted specification of its performance; the axial anechoic frequency response, by which prior art systems are characterized. This has become the single most important factor in the marketing of loudspeaker systems for home reproduction.
The sound field generated at a live performance has two principal components: a direct field radiated from the performer directly to the listener (or microphones) and a reverberant field formed by reflections, primarily from the boundaries behind and to the sides of the performer. The direct field diminishes in intensity as the inverse of the square of the distance between the listener and the performer. The reverberant field establishes, by multiple reflections, a consistent level throughout the hall proportional to the intensity of the source. As the distance from the performer to the listener increases, the level of the direct field rapidly decreases, until it falls below the level of the reverberant field. In real life, the listener at a live performance is almost always in an area in which the reverberant field predominates over the direct field, and that reverberant field originates primarily from the same direction as the direct field and secondarily from surfaces near the listener, and it is delayed by approximately one millisecond for each foot of the reverberant field path length greater than the direct field path length. The listener subconsciously registers the time difference between the two fields, and from that difference two very important conclusions can be drawn:
First, the direction from which the sound orginates is determined by a phenomenon known to psychoacousticians as "The Precedent Effect." This stipulates that the first wave form in a toneburst establishes the direction of origin of the tone. This phenomenon provides us with the mechanism for stereophonic hearing. It is unambiguous until the intensity of the later arriving reverberant field is approximately ten times that of the first arriving direct soundfield.
Secondly, the aural ambience of the performance hall results from one's subconcious comparison of the direct and reverberant soundfields. It establishes for the listener an appreciation of the size of the hall and its acoustical texture and gives the listener an appreciation that the hall is filled or empty.
In theory, the microphone replaces the listener in the hall, and the loudspeaker reproduces exactly what the microphone "hears". At this point, the directionality of conventional loudspeakers creates problems which become worse as the frequencies go higher. A conventional loudspeaker system does not reproduce sound with the same distribution pattern that is derived from a live performance: it radiates energy in a directional pattern that varies with frequency, due to mass control of the driver at the higher frequencies.
The home listening room is, almost without exception, smaller than the site of the original performance. Such a room has its own aural ambience, made up of the direct sound field from the loudspeaker to the listener, and a reverberant sound field created by multiple reflections, primarily from the boundaries behind the listener instead of from behind the performer, as in the case of a live performance.
The psychoacoustic effect of this reversal of the apparent direction of the origin of the reverberant sound field is in the recognition that such field reversals are indicative of the sound of loudspeakers. This is one of the two major problems affecting our perception of reality in home music systems. The other one is the severe inadequacy of treble (high frequency) response in musical reproduction in the home listening room. A conventional loudspeaker system sold on the basis of a flat axial anechoic frequency response curve will actually achieve good high frequency performance only on axis in an anechoic, or reflection-free, environment. Prior art loudspeakers are actually used in home listening rooms which are not anechoic. They are invariably quasi-everberant, and integrate all the sound energy radiated into the room. The result is a performance that is more accurately depicted by a total energy frequency response curve in which there is a high frequency deficiency. The reason this high frequency deficiency exists is that mass control of the drivers causes audio energy to decrease when the force required to maintain output exceeds the magnetic flux available in the gap. Energy is maintained along the central axis but for all other angles the sound pressure level decreases rapidly. Sound played in a quasi-reverberant room-any room with floor, walls, and a ceiling-is integrated to its average level in the reverberant sound field. The reverberant sound field is much more intense than the direct sound field at normal listening distances. It therefore becomes evident that the directionality of conventional loudspeaker systems makes the achievement of a good balance between treble and mid-range or bass portions of the aural spectrum a virtual impossibility.
Clearly then, only a loudspeaker system which radiates all its acoustic energy equally along every radial axis throughout 360.degree. in the horizontal plane will generate an aural ambience in which the reverberant sound field is formed by reflections originating primarily from the same direction as the source--from the boundaries behind the loudspeaker--and therefore the apparent source of the sound field is from the direction of the loudspeaker. This distribution is typically characteristic of the radial loudspeaker, and accounts for the superior spatial perspective of such systems.
An unconventional loudspeaker system that avoids the problems of distorted spatial perspective and spectral imbalance and which is capable of home music reproduction of sound that gives a listener the perception of reality--a conviction that he is present at an original performance--must have the following characteristics:
1. The sound energy from the loudspeaker system must be radiated equally on all axes through 360.degree. in the horizontal plane. PA0 2. Electroacoustic conversion efficiency must be relatively equal for all frequencies within the band-pass of the loudspeaker. PA0 3. The loudspeaker system must be capable of relatively distortion-free reproduction when played at tutti-fortissimo orchestra levels in a listening room of the volume for which it has been designed. PA0 4. The loudspeaker system must minimize distortions and coloration.
The 360.degree. radial orthospectral tweeter assembly of this invention achieves the distribution of all its acoustic energy uniformly through 360.degree. in the horizontal plane, and radiates that energy with effectively equal spectral efficiency. It is compact, cost effective, and extremely low in unwanted coloration.
The high frequency loudspeaker of this invention includes a unique application of horn-loading technology to a conventional dome diaphragm dynamic driver or ring driver. Lateral sound distribution in this invention is controlled by the use of a reflective wave-guide and provides virtually uniform efficiency by controlling the acoustic coupling between the mouth of the horn and its throat. It overcomes the decrease in total acoustic energy output at the higher frequencies characteristic of all prior art electroacoustic transducers, caused by the effects of mass control of the diaphragm. This acoustic equalization is possible because of the much higher conversion efficiency of the horn-loaded driver when compared to its direct-radiator equivalent.
A number of 360.degree. horn devices were patented in the early years of radio. All are of pragmatic design and execution, obviously intended to provide a uniform sound field for a number of listeners sitting around a table mounted horn. In all devices of this type, the bending of the sound energy from the vertical axis of the flat diaphragm type driver to the horizontal plane was accomplished by refraction and diffraction. The loss of high frequency response in those older systems was immaterial because the program material lacked high frequencies. Later devices intended for the burgeoning public address systems market used horn configurations of significantly higher efficiency and acoustic power capability. But these also depended on refraction and diffraction to achieve dispersion. Their high frequency responses were adequate for the limited bandpass of the early public address systems.
In the present invention a format is created by which the higher frequencies are bent from the vertical axis of the driver to the horizontal plane for uniform polar distribution without loss of the higher frequencies. This is accomplished by using closely controlled reflection to effect the change of direction, rather than the refractive bending used in prior art horns. For a convex spherical dome used as a compression driver, the reflective means is an frusto-conical surface whose apex is on the central axis of the dome diaphragm.
The cross-sectional area of the throat of the horn is uniformly distributed around the vertical axis of the tweeter assembly. The upper surface of the horn is a frustoconical reflective surface upwardly angled from the inner edge of the throat diverging from the driver's central axis by half the difference between 90.degree. and the throat's centerline divergence from that axis. This satisfies the requirement for undistorted reflection; the angle of incidence of a wave-front originating along the throat centerline is equal to the angle of reflection. The mouth of the horn, looking backward toward the throat, will "see" a virtual image of the throat at a distance behind the reflecting surface equal to the distance of the throat below that surface.
In the present invention a flare rate of the horn may be conical, or have manifold exponential sections resulting in a conical-like rate. This principle is used in a well-known prior art loudspeaker system to increase the efficiency at the top end of a folded exponential bass horn effected by using dual flare rates.
This principle is applied to the novel radial horn of the present invention but its scope and effect are more extensive. The flare rate must be calibrated to effect the degree and scope of equalization required to achieve uniform power response. The horn area must increase at the chosen rate of flare until the horn mouth area equals a circular horn whose diameter is one-quarter wavelength of the crossover frequency. The low frequency cutoff of the horn should be at a frequency slightly higher than the free-air resonance of the driver.